1. Field of the Invention
The invention generally relates to surge protection of coaxial cables and transmission lines. More particularly, the invention relates to a planar coil surge suppressor insert and related compact surge protector housing for use in-line with a coaxial cable or transmission line, configurable for a range of different frequency bands.
2. Description of Related Art
Electrical cables, for example coaxial transmission lines of antenna towers, are equipped with surge suppression equipment to provide an electrical path to ground for diversion of electrical current surges resulting from, for example, static discharge and or lightning strikes.
Prior coaxial suppression equipment typically incorporated a frequency selective shorting element between the inner and outer conductors dimensioned to be approximately one quarter of the frequency band center frequency in length, known as a quarter wavelength stub. Therefore, frequencies within the operating band pass along the inner conductor reflecting in phase from the quarter wavelength stub back to the inner conductor rather than being diverted to the outer conductor and or a grounding connection. Frequencies outside of the operating band, such as low frequency surges from lightning strikes, do not reflect and are coupled to ground, preventing electrical damage to downstream components and or equipment.
Depending upon the desired frequency band, a shorting element dimensioned as a quarter wavelength stub may have a required dimension of several inches, requiring a substantial supporting enclosure. Prior quarter wavelength stub surge suppressors, such as described in U.S. Pat. No. 5,982,602 “Surge Protector Connector” by Tellas et al, issued Nov. 9, 1999 commonly owned with the present application by Andrew Corporation and hereby incorporated by reference in the entirety, reduce the required enclosure size by spiraling the stub within the enclosure, forming a planar coil normal to the inner conductor.
To avoid undesired parasitic capacitance and or resonant effects between the initial coil winding and the inner conductor, the coil is loosely wound to increase the spacing between the coil and the inner conductor. To enclose the planar coil, the required enclosure is relatively large and expensive. The increased overall diameter of the required enclosure spaces interconnection cables away from cable runs, because the inner conductor minimum distance from a mounting plane such as a wall is increased as the enclosure diameter increases. Also, because the planar coil is normal to and wound about the inner conductor, the assembly must be removed from connecting cables and or equipment to enable disassembly of the surge suppressor for inspection and or exchange of the planar coil.
As the spiral aspect of the shorting element increases, an inductance arises. The high frequency magnetic field effects of an inductor structure having an affect on the impedance of the frequency selective shorting element that allows the overall length of the shorting element to be reduced, compared to a straight or loosely spiraled quarter wavelength stub. U.S. Pat. No. 6,452,773 “Broadband Shorted Stub Surge Protector” by Aleksa et al, issued Sep. 17, 2002 commonly owned with the present application by Andrew Corporation and hereby incorporated by reference in the entirety applies a stub portion and an inductor portion formed as a helical rather than planar coil. Although the combination of a stub portion and an inductor portion widens the operating frequency band of the device, different frequency band specific shorting element configurations may still be required to satisfy specific frequency bands. The helical coil inductor portion may be accessed with minimal disassembly, but requires precision machining operations during manufacture and a corresponding elongated enclosure cavity.
Competition within the electrical cable and associated accessory industries has focused attention on cost reductions resulting from increased manufacturing efficiencies, reduced installation requirements and simplification/overall number of discrete parts reduction.
Therefore, it is an object of the invention to provide an apparatus that overcomes deficiencies in the prior art.